1. Field of the Invention
The present invention relates to a diffraction optical element and a production method thereof, particularly to the diffraction optical element having an element formed in a staircase shape and the production method thereof.
2 . Description of the Related Art
Recently, there is an increasing demand for the diffraction optical element in which a traveling direction and a phase of light are controlled by a periodic fine structure. There are various kinds of shapes of the diffraction optical elements. The diffraction optical element whose section is formed in a sawtooth shape has high diffraction efficiency in theory. However, actually the diffraction optical element having a staircase shape which is approximate to the sawtooth shape is frequently used because the staircase shape is easy to produce. Generally there is well known a method of producing the diffraction optical element having the staircase shape, in which a semiconductor fine process technology is utilized and the 2m-step diffraction optical element is produced by repeating a series of processes of exposure, development, and etching with m (m is a natural number) mask patterns (for example, see Japanese Patent Application Laid-Open No. 11-14813 and H. Sasaki et al., “High-accuracy packaging technology for light source and silicon micro-lens”, Trans. of Japan Institute of Electronics Packaging, Vol. 5, No. 5, p. 466-472 (2002)).
In the diffraction optical element production method, for the case where a patter having a minimum line width thinner than resolving power of photolithography is produced, there is well known a method of decreasing an eight-step staircase shape to a four-step or two-step staircase shape to widen a step width as shown in FIG. 11A (for example, see Yuko Orihara, Werner Klaus, Makoto Fujino; and Kashiko Kodate, “Optimization and application of hybrid-level binary zone plates”, Appl. Opt. 40, 5877-5885(2001)).
In this case, when a lens curved surface is approximated by the eight-step staircase shape and the four-step staircase shapes, a shift of a light phase is generated at a boundary between the eight-step staircase shape and the four-step staircase shapes, which results in degradation of optical characteristics of the lens. In order to prevent the degradation of the optical characteristics, Yuko Orihara, Werner Klaus, Makoto Fujino, and Kashiko Kodate, “Optimization and application of hybrid-level binary zone plates”, Appl. Opt. 40, 5877-5885(2001) discloses a method of adjusting a step (height) or a width (distance) of the staircase shape to perform phase correction.
FIG. 11B shows the phase correction for adjusting the step. In FIG. 11B, the height of the four-step staircase shape is corrected in the direction toward the lower step by λ/16, and the height of the two-step staircase shape is further corrected by λ/8. FIG. 11C shows the phase correction for adjusting the width. In FIG. 11C, the four-step staircase shape is corrected in the direction toward the eight-step staircase shape by λ/18, and the two-step staircase shape is further corrected in the direction toward the four-step staircase shape by λ/4 (λ/8+λ/4 in total).
However, there are the following problems in the phase correction shown in FIGS. 11B and 11C.
(1) Phase Correction by Step
In the eight-step staircase shape, the staircase shape is usually produced by etching of depths of λ/8, λ/4, and λ/2 . However, because it is necessary to form the step of λ/16 at the boundary between the eight-step staircase shape and the four-step staircase shape, it is necessary to add an etching process. Further, due to the additional etching process, it is necessary to add the one or two mask patterns.
(2 ) Phase Correction by Width
In order to eliminate the boundary between the eight-step staircase shape and the four-step staircase shape or the boundary between the four-step staircase shape and the eight-step staircase shape, as shown in FIG. 11C, it is necessary to form thinner line width. Because the switch portion between the staircase shapes having the different step numbers is formed by a resolution limit portion, it is actually difficult to produce the portion having further thinner line width.